1. Field of the Invention
The present invention relates to a master cylinder for use in a fluid-pressure brake device in a motor vehicle, and more particularly to a master cylinder for discharging a large quantity of fluid under low pressure during an initial period of its operation stroke and for successively producing high fluid pressure.
2. Description of the Prior Art
One known master cylinder of the type described is disclosed in U.S. Pat. No. 4,208,881. As shown in FIG. 1 of the accompanying drawings, the disclosed master cylinder includes a cylinder body having a stepped bore formed with a larger diameter section B and a smaller diameter section C. A piston D slidable in the stepped bore is of a stepping piston having a larger diameter head E and a smaller diameter head F. The larger diameter bore section B defines therein a first pressure chamber G and the smaller diameter bore section C defines therein a second pressure chamber H. When a negative pressure is developed in the first pressure chamber G, a first valve I is opened to allow a fluid to flow from a reservoir J through a port K into the first pressure chamber G. When the pressure in the first pressure chamber G exceeds a predetermined level, a second valve L is opened to draw the fluid into the reservoir J. The pressure difference between the first and second pressure chambers G, H causes the fluid to flow from the first pressure chamber G into the second pressure chamber H around the outer peripheral lip of a cup seal M mounted on the smaller diameter head F of the piston D.
The first valve I is closed when a peripheral wall Na of a member N made of rubber or other elastomeric material is sealingly engaged by an inner peripheral wall surface of a coupling hole Q in the cylinder body A. As a negative pressure is created in the first pressure chamber G, the peripheral wall Na is elastically deformed to permit the fluid to flow from the reservoir J into the first pressure chamber G through a gap formed between the peripheral wall Na and the inner peripehral wall surface of the coupling hole Q. When the ambient temperature around the master cylinder is lowered, the member N of rubber or other elastomeric material is hardened to the extent that even when the negative pressure in the first pressure chamber G reaches the predetermined level, the first valve I is not opened with the resulting danger of admitting air into the first pressure chamber G. To prevent such a problem from occurring, it has been necessary to provide a complete seal between a seal member P fitted over the larger diameter section E of the piston D and an inner peripheral surface of the larger diameter bore section B. Accordingly, the parts are required to be of a high accuracy, and the seal member P has to be of a special design.